1. Field of the Invention
The present invention relates to a mobile communication system, and in particular to a method for processing abnormal handover termination in a universal mobile telecommunications system (UMTS).
2. Description of the Background Art
Radio communication for transmitting/receiving data packets or different type of information by using radio contact techniques and cellular type mobile communication techniques is the backbone of information society through far-reaching business use and non-business use applications. Hereinafter, this concept is simply called “mobile communication”.
In general, a mobile communication system can be constructed as an analog cellular system, a DCS (digital cellular system), a PCS (personal communication services), a GSM (global system for mobile communications) and an IMT-2000, etc. Herein, these systems use an air interface standard such as a CDMA (code division multiple access), a TDMA (time division multiple access) and a FDMA (frequency division access).
The mobile communication system is classified into a first generation analog type system, a second generation digital type system (2G) and a third generation upgraded digital type system (3G).
Herein, two standards used in the second generation mobile communication system are a GSM system using the TDMA as an air interface technique and a CDMA system using the CDMA as an air interface technique.
In addition, the IMT-2000 or an UMTS (universal mobile telecommunications system) is known as a third generation mobile communication standard. Two standard establishment bodies, namely, 3GPP (third generation partnership project) and 3GPP2 (third generation partnership project two), provide these third generation standards. Various betterment of the standardized second generation mobile communication system are reflected in the third generation standards.
FIG. 1 is a schematic view illustrating a general UMTS.
As depicted in FIG. 1, the UMTS system includes a RNC (radio network controller) 100, plural Node Bs (base stations) 200-1˜200-n managed by the RNC 100, and a CN (core network) 300.
The RNC 100 allocates and manages radio resources, and performs circuit exchange communication with the GSM network and packet exchange communication with the GPRS network by being operatively coupled to the CN 300.
The Node Bs 200-1˜200-n receive information transmitted from a physical layer of an UE (user equipment) 400 and performs an access point function of the UE 400 transmitting data to another UE 400.
A handover operation in the general UMTS system will be described.
In general, in the mobile communication system, a service region is divided into plural cells. Accordingly, the UE 400 performs communication by setting at least one call while passing an overlap region between adjacent cells.
The RNC 100 transmits parameters necessary to handover determination such as a reporting range, a hysteresis, a time trigger, etc., to the UE 400 by using a RRC (radio resource control) message. Accordingly, the UE 400 receiving the RRC message finds a measurement quantity of communicating cells and a measuring cell, and determines handover by using the received parameters.
In more detail, as depicted in FIG. 2, when a measurement quantity of the very best cell among the cells communicating with the measuring cell is less than the hysteresis, the UE 400 adds a cell (event A: adding Cell 2). When a measurement quantity of the very bad cell among the cells communicating with the measuring cell is greater than the hysteresis, the UE 400 replaces the cell (event C: replacing Cell 1 with Cell 3). When a measurement quantity of the very best cell among the cells communicating with the measuring cell is greater than the hysteresis, the UE 400 removes the cell (event B: removing Cell 3).
Accordingly, as depicted in FIG. 3, a RRC (radio resource control) layer 10 (hereinafter referred to as a RRC) of the UE 400 analyzes the measurement quantity found at the physical layer of the UE 400 and checks whether the pertinent measurement quantity satisfies the above-described event condition for handover.
If the event condition for handover is satisfied, the RRC 10 of the UE 400 requests a measurement report message transmission to a RLC (radio link control) layer 20 (hereinafter referred to as a RLC) of the UE 400. The RLC 20 receiving the measurement report message transmission request transmits a measurement report message to the RNC 100 by using an AM (acknowledge mode) method.
When the measurement report message is received, the RNC 100 transmits a reply signal (RLC_ACK) for indicating normal reception of the measurement report message to the RLC 20 and analyzes the measurement report message. As a result of this analysis, if it is judged that the event condition for handover is satisfied normally, the RNC 100 transmits an active set update message as a handover approval message to the RLC 20 of the UE 400, and accordingly the handover procedure starts.
The RRC 10 performs the handover by processing the active set update message received through the RLC 20. When a radio link setting of the UE 400 is normally adjusted (added or deleted) through the handover, the RNC 100 requests an AM type transmission of an active set update complete message to the RLC 20. Accordingly, the RLC 20 transmits the active set update complete message to the RNC 100 according to the request of the RRC 10 and waits for a certain time duration until a RLC_ACK (reply signal) is received from the RNC 100.
In the meantime, the RNC 100 completes the handover procedure by performing an additional handover setting according to the active set update complete message transmitted from the RLC 20 and transmits the RLC_ACK to the RLC 20.
However, when the RLC_ACK from the RNC 100 is not received by the RLC 20 after the certain time has elapsed, the RLC 20 retransmits the active set update complete message to the RNC 100 in order to receive the RLC_ACK from the RNC 100. Afterward, when the RLC_ACK is received from the RNC 100, the RLC 20 transmits to the RRC 10 a confirmation signal indicating that it performed the active set update complete message transmission properly. Accordingly the RRC 10 completes the handover procedure properly.
However, in the conventional handover method, when the RLC_ACK is not transmitted from the RNC to the RLC due to an error occurrence in a down-link channel, the RLC cannot transmit to the RRC a needed confirmation signal indicating that it performed properly the active set update complete message transmission. Thus, without the receipt of the confirmation signal from the RLC, the RRC cannot complete the handover procedure properly. Accordingly, problems such as call interruptions in communications occur.